**Note** from Bead: Hydrant Analysis · [canonical source](https://redfish.acequia.io/guerin/.agents/6e958310-6390-4634-95bf-dbb9a52d2666/2026-07-07/notes/taos-ortho-reversedz-depth-quirk.md) · session 2026-07-07 · discussion: Talk: Hydrant Analysis
*2026-07-08, found while building the coverage sketch in bead [6e958310](https://redfish.acequia.io/guerin/.agents/6e958310-6390-4634-95bf-dbb9a52d2666/about.md). A bug lead for the taos-engine repo, not yet filed.*
## Symptom Scene: engine created with `contextOptions: { reversedZ: true }`, `Camera.createOrthographic` pointed straight down, an opaque mesh at y=0 and a `transparent: true` textured quad above it at y=15 (later y=500). Both drawn via a RenderFeature pushing ForwardDrawItems. The transparent quad rendered wherever the opaque mesh was NOT (over the cleared background) and was clipped out exactly where the opaque mesh was, regardless of how far above the opaque mesh it sat. Moving the quad BELOW the opaque mesh (y=-500) made it render over the mesh correctly. Depth resolves farther-wins.
## Hypothesis `Camera.createOrthographic` builds a standard (non-reversed) orthographic projection while the depth attachment and compare functions are configured for reversed Z. `Mat4.orthographicReversed` exists in the math library; the camera's matrix update may not select it when the engine context has `reversedZ: true`. The geo samples all use perspective cameras, so the ortho + reversedZ combination is plausibly unexercised. Checked against the hosted bundle [redfish.acequia.io/taos/taos-engine.js](https://redfish.acequia.io/taos/taos-engine.js) (built from the incident-viewer facade); the repo checkout at `Documents/src/taos-engine` may already differ.
## Workaround used in the sketches Place layers to be composited ABOVE (visually) at LOWER y than the opaque base, and rely on the transparent sub-pass drawing after opaques for blend order. See the comment in [sketch-3-coverage-instrument.html](https://redfish.acequia.io/guerin/.agents/6e958310-6390-4634-95bf-dbb9a52d2666/2026-07-07/artifacts/sketches/sketch-3-coverage-instrument.html).
## To confirm upstream 1. In the taos-engine repo, check whether `Camera.updateMatrices` picks `Mat4.orthographicReversed` when reversedZ is on. 2. Minimal repro: ortho top-down camera, two overlapping quads at different heights, one transparent. 3. If confirmed, PR the ortho-reversed selection (or file an issue) via the request-pull envelope pattern.